Detector amplifier circuit



July 27, 1937.` c, J, RANKS 2,088,168

DETECTOR AMPLIFIER CIRCUIT Filed June 22, 1935 5 Hf AMPA/HER /r 7 Y 70 2 AVC lNvENToR CHRISTOPH? J. FRANKS ATTORNEY Patented July 27, 1937 UNITED STATES DETECTOR. All/IPLIFIER CIRCUIT Christopher J. Franks, Boonton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 22, 1935, Serial No. 27,842

5 Claims. Y (Cl. Z50-27) My present invention relates to signal detector and audio amplier networks, and more particularly to improved methods of, and circuits for, detecting and amplifying in a single electron discharge tube stage. l

One of the primary objects of my invention is to provide a multi-function tube circuit capable of detecting signals and amplifying the audio component of the detected signals; the circuit having associated therewith a device for insuring amplication ofthe audio component without distortion even with incoming signals of normally overloading intensity.

Another important object of the invention is to provide a, detector-audio amplifier stage of the coplanar grid tube type; the stage including an auxiliary electron discharge tube which functions to amplify and faithfully repeat that half of the modulation envelope of the detected signals which is not repeated by the co-planar grid tube at grid overloading signal intensity.

v Another object of the invention is to improve the operation of tube circuits adapted to simultaneously detect signals, amplify the audio component of detected signals and produce gain control bias for signal amplifiers; the improvement residing in the provision of a repeater tube whose sole function it is to repeat into a common output circuit that half of the audio wave which is not amplified by the main circuit because of a blocking action caused by signal waves of relatively high intensity.

Still other objects of my invention are to improve generally the efficiency and reliability of co-planar grid tube detector circuits, and more especially to provide multi-function tube circuits which are readily constructed and assembled in radio receivers.

'I'he novel features which I believe to be charac teristic of my invention are set forth in particularity in the appended claims, the invention itself,\.

however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagirammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:-

Fig. 1 shows a circuit embodying a form of the invention,

Fig. 2 illustrates a modification thereof.

Fig. 3 shows another modification.

Referring now to the accompanying drawing, wherein like reference characters in the different Q5? figures designate similar circuit elements, the receiver shown in Fig. 1 is of a conventional type and embodies a signal collector A; an R. F. amplifier I; a detector-amplifier 2; and an auX- iliary tube 3 whose function will be described later. The audio output of tube 2 is impressed through coupling transformer l upon one, or more, later stages of audio amplification; and a final reproducer completes the receiver system. The network I may comprise one, or more, tunable radio frequency amplifiers, and variable condenser 5 is to be understood as denoting the usual gang tuning condenser. In this type of receiver (t. r. f.) the variable tuning condenser ii, of the detectorarnplier tube 2, has its rotors ganged with the rotors of condenser 5 (as shown by theI dotted lines). Of course, the signal circuits are all tuned to a common, desired signal frequency.

If the receiver is of thesuperheterodyne type, the network I comprisesk the usual radio frequency amplifier; local oscillator; first detector; and intermediate frequency amplifier.` In such a case the condenser 6 would be fixed, and resonate thedetector input coil I to the operating I. F. frequency. The tube 2 and its associated circuits function as the second detector if the system is a superheterodyne receiver. Such receiving systems (t. r. f. and superheterodyne) are too Well known to those skilled in the prior art to require further description.

Regardless of the type of receiving system used,

the tube 2 is shown as being of the co-planar grid type. The characteristics and functioning of such type of tube have been described, and claimed," by K. Posthumus in United States Patent No.

1,986,851 issued January 8, 1935. The tube inr cludes a cathode 8, anode 9 and a pair of signal control grids i6 and II. These two grids are coplanarly wound, and are connected to opposite ends of Vinput coil` l. Thus, the two grids-are at radio frequency potentials of opposite phase at any given instant. The anode-9 surrounds the grids I il and I I, and for this reason the radio frequency effect on the electron stream flowing into the plate circuit, ldue to the carrier potential variations on the two grids, is substantially suppressed.

Rectiflcation is secured across the resistor I2 connected between the grounded cathode and the mid-point of coil l. The auxiliary tube 3 has its cathode grounded, while its plate I3 is connected to one end of the primary Il of output transformer 4; the anode 9 is connected to the opposite end of primary Il. A source B of positive voltage has its negative terminal grounded; the positive side thereof is connected to the mid-point of primary 4. The input grid I4 of tube 3 is connected to the grid side of resistor l2 through an auto-transformer I5. An AVC (automatic volume control) lead I6, including a pulsating current suppressor resistor H, is connected from a tap on reactor l5 to 'the grids of the gain-controlled ampliers in network l. An audio frequency by-pass condenser IS of about 0.5 rnfd., is connected to ground from the tap point on reactor i5. Those skilled in the art are well aware of the function of the AVC arrangement, and know that it acts to maintain the signal carrier intensity at the dernodulator input substantially constant over a wide range of carrier amplitude variation at the collector A. This is accomplished by increasing the negative bias on the controlled ampliiier grids as the received carrier amplitude rises.

Considering, now, the operation of tube 2 and its auxiliary tube 3, it is first pointed out that tube 2 functions in the following manner. The radio frequency signal potentials are applied to control grids Il] and Il in reverse phase, as pointed out heretofore, when it is desired to detect incoming signals. The rectified audio frequency currents appear across the resistor i2, and the audio frequency component is applied to the two grids I0 and ll in parallel. The applied audio component is amplified and repeated by tube 2 functioning as an audio amplifier; the repeated audio wave appears in the circuit of anode 3, and is then used to excite the following power ampliiier. Automatic volume control bias for the preceding stages to be controlled in gain-is obtained by connecting the AVC lead I6 to a point on the resistor i2 which is at a negative direct voltage with respect to the potentials of the cathodes of the controlled stages, and the latter cathodes may be grounded as is usual in ycornrnon practice.

One of the principal advantages of the coplanar grid tube used as a detector is that since at any instant the radio frequency potentials appearing on the two control grids are equal and opposite the net effect on the plate current is zero, and substantially no radio frequency component appears in the plate circuit of tube 2. As a consequence no audio frequency current due to plate rectification action can appear in the plate circuit of the co-planar grid detector tube. Therefore, more audio frequency output can be obtained from this type of circuit than from the conventional type. This has been clearly disclosed in the aforesaid Posthumus patent.

The co-planar grid detector tube characteristic has a longer linear portion than the conventional detector tube circuit, and consequently a greater output at overload, as well as a substantially distortionless output at lower outputs. Overload is reached when signal inputs are sufcient to swing the plate current over the lower curved portion of the tube characteristic, and in the cut-olf region. If the carrier input is increased sufficiently to bring the main operating point down on the foot of the characteristic curve of the tube, then only one-half of the modulation envelope will appear in the plate circuit, the other half corresponding to a grid voltage swing to the right of the main point, which is, of course, in the cut-olf region.

In other words, when the operation of tube 2 is considered without any reference to the embodiment of the present invention, it will be seen that since the control grids l0 and Il are connected to the negative side of resistor l2, there will finally be reached a signal intensity level at which the control grids Iii and il will be negatively biased to a point such that plate current flow will be substantially cutoff. Such cut-off occurs during the negative half of the audio swing, and for this reason only the positive half of the audio wave will be amplified, while the negative half will not be repeated into the plate circuit. This will, oi' course, give rise to serious distortion effects. It is pointed out that this action is very similar to what occurs in audio amplifiers of the class B type.

According to the present invention, the auXiliary tube 3 is introduced into the circuit in the manner shown in Fig, l, and the function of tube 3 is to amplify that half of the audio wave which is not repeated into the plate circuit of tube 2. Thus, by connecting the plate circuits of tubes 2 and 3 in push-pull output connection the original input wave characteristic is obtained in the common output circuit of tubes 2 and 3. The auto-transformer i5 is connected between the signal input grid i4 of tube 3 and the negative side of resistor I2, and the transformer functions to reverse the audio frequency phase for application to the grid of tube 3. The introduction of the auto-transformer l5 in n0 way disturbs the AVC action, since the AVC lead i8 is tapped to the reactor I5.

The tube 3 is not concerned with rectification in any way, being used merely to amplify the reverse half of the audio frequency cycle. OI course, the tube 3 could also be used to rectify. In such case the circuit associated with tube 3 would have to include coupling means for keeping the control grids of the second tube at the same radio frequency potentials, but at reverse audio frequency phase with respect to the control grids of tube 2. Again, a tube circuit can be used as a phase reverser instead of the auto-transformer.

Fig. 2 shows a variation of the embodiment shown in Fig. l. It will be observed that in Fig. 2 the auto-transformer l5 is connected directly between the mid-point of input coil 'l and the signal grid I4 of auxiliary tube 3. The rectification resistor I2 is connected between the AVC lead and ground, the AVC lead being connected to a tap on the transformer l5. An audio frequency by-pass condenser 20, of 0.5 mfd., is connected in shunt across resistor l2. The arrangement shown in Fig. 2 functions in a manner similar to that described in connection with Fig. l.

It is pointed out that the present invention is not restricted in any manner to the use of tubes of the co-planar type. For example, a tube of the 55 type, also known as a duplex diode triode, could readily be employed in place of tube 2 in Fig. 1. In such case, and as shown in Fig. 3, the resistor l2 is the diode load resistor connected between the cathode and the diode anodes of the tube 55. The control grid 55 of the triode section of the 55 tube is then connected to the diode anode side of the diode load resistor. The circuit is otherwise the same as that shown in Fig. 1. The value of the present invention in connection with a 55 tube resides in the fact that the cut-off action occurring with relatively high signal inputs is compensated for by the auxiliary tube 3.

While the prevention of overloading may be accomplished by designing the AVC' properly; yet the present invention, as applied to the three figures, is useful in cases where the AVC is inefficient, or where none is used. Of course, if the characteristics of the detector are such as to overload on very low signals, as when the ampliiication factor of the detector tube is high, then the invention is also useful.

While I have indicatedand described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim ist 1. In combination with an electron discharge tube provided with at least a cathode, a grid, an output plate and an auxiliary cold electrode, a path connecting the cathode and auxiliary electrode and including a signal input circuit and a resistor across which is developed a direct current potential from rectied signal current flowing through the resistor, a connection between said grid and a point on said resistor which assumes a negative direct current potential with respect to the cathode when signals are impressed on said input circuit, the audio component of rectified signal current flowing through the resistor being impressed on the grid, an audio output circuit connected to the output plate, a second tube having at least a cathode, input grid and output plate, means connecting the last grid to said point, said last plate being connected to said outputcircuit, and said connecting means being constructed and arranged to reverse the phase of the audio component impressed on the said grid of the second tube.

2. In a detector comprising a tube which is provided with a cathode and anode between which there exists a single electron path, a signal input circuit, a plurality of means, each aiecting substantially the entire electron path, connected between said input circuit and said path creating ,a potential iield in thepath whose intensity is proportional to the strength of received modulated signal energy, the connections to the input circuit being chosen in such a manner that sig- Y nal potentials are impressed on the electron path in opposing phase, the improvement which includes an electron discharge device having an input electrode and an output electrode, an audio cycle reversing path connecting the input electrode to said signal input circuit, and the output electrode being connected to the anode of said detector tube to provide a common output circuit.

3. A signalY receiver network comprising, in combination, an electron discharge tube including a cathode and anode between which there exists a single electron path, a signal input cirinput grid to said input circuit, and the anode of said auxiliary tube being connected to the anode of the first tube to provide a common output circuit.

4. A signal receiver network comprising, in combination, an electron discharge tube including a cathode and anode between which there exists a single electron path, a signal input circuit, a pair of co-planarly arranged grids in said path, connections between said input circuit and said grids whereby a negative potential is created in the path proportional tothe strength of the signals, said connections being made to such radio frequency potential points on the input circuit that signal potentials are impressed on said path in opposing phase, and an auxiliary tube including a cathode, input grid and anode, an audio wave reversing reactor connecting the input grid to said input circuit, and the anode of said auxiliary tube being connected to the anode of the first tube to provide a common output circuit, said connections between said input circuit and said coplanar grids including a resistor, and an automatic volume control lead connected to a point on said resistor which is at a negative direct current voltage.

5. In a circuit as dened in claim 1, said aux'- iliary electrode being an anode disposed adjacent said cathode but outside the electron stream to the grid and plate.

CHRISTOPHER J. FRANKS. 

